1. Field of the Invention
The present invention relates to an exhaust emission purifying apparatus for an engine that reduces and purifies nitrogen oxides (NOx) in exhaust gas using a reducing agent.
2. Description of Related Art
As an exhaust emission purifying apparatus that purifies NOx contained in exhaust gas from an engine, an exhaust emission purifying apparatus using ammonia as a reducing agent was known. For example, an exhaust emission purifying apparatus disclosed in Japanese Patent Application Publication No. 2000-27627 injection-supplies a urea aqueous solution to an upstream side of a NOx reduction catalytic converter positioned in an exhaust passage, to reduce and purify NOx in exhaust gas by ammonia that is generated from the urea aqueous solution.
However, since the conventional exhaust emission purifying apparatus mentioned above injection-supplies the urea aqueous solution when an increase in NOx concentration in the exhaust gas is detected, there was a possibility that, for example in the case in which an engine operating state changes suddenly by a driver's shift operation or the like and an NOx amount emitted from the engine increases, a control delay occurs and a sufficient amount of urea aqueous solution may not be injection-supplied, resulting in a decrease in NOx purification rate.